The present invention generally relates to an image sensor, and in particular to an image sensor such as a charge-coupled device image sensor having charge storage regions. More particularly, the present invention relates to an image sensor capable of detecting a greatest charge amount and a smallest charge amount in real time by separately monitoring potentials of floating electrodes formed above respective charge storage regions. The present invention is suitably applied to a camera and so on.
As is well known, a solid image sensor such as a charge-coupled device (hereinafter simply referred to as a CCD image sensor) is widely used in various fields of a facsimile machine, a camera and so on. A conventional CCD image sensor has a function of only transferring stored charges which are generated by irradiation of light and which are then stored in charge storage regions. Such a CCD image sensor can be used in ah application in which the intensity of light to be processed is limited within a certain range, such as a facsimile machine. However, the CCD image sensor having only the charge transfer function, is not suitable for an application in which a dynamic range of light in intensity to be processed is great, such as a camera. For example, the amount of light applied to a photodiode built in the CCD image sensor may become excessive due to strong illuminance of an object. In this case, a charge storage region overflows with a stored charge. On the other hand, when the amount of light received by a photodiode is insufficient, the sensitivity of the image sensor is decreased, and thereby the image sensor do not sufficiently functions as an image sensor.
In order to overcome the above problems, the applicant has proposed an improved CCD image sensor in the Japanese Laid-Open Patent Application No. 61-226953. The proposed image sensor has an automatic gain control function, in which a time during which a charge is stored in a charge storage region, is determined by sensing an optical state of a CCD image sensor in real time. The optical state of the CCD image sensor is obtained by detecting an average in amounts of the charges which are generated by the respective photodiodes, and are then stored in the respective charge storage regions of the CCD image sensor. The storage electrodes (floating electrodes) formed above charge storage regions are mutually connected. A potential of the mutually connected floating electrodes is monitored. The potential changes, depending on the amount of light received by the CCD image sensor. The measured potential corresponds to the average amount of the charges stored in the charge storage regions. When the potential of the mutually connected floating electrodes exceeds a reference potential, the CCD image sensor is considered being about to overflow. Therefore at this time, a charge transfer operation is activated.
However, the proposed image sensor has disadvantages described below. The optical state of the CCD image sensor is monitored by obtaining the average amount of charges stored in charge storage regions. Therefore, the average amount of charges may be smaller than the reference level, through a charge storage region overflows with excessive charges. Further, the average amount may be kept almost constant, through the greatest charge amount and/or the smallest charge amount changes every moment. In other words, the average of the amounts of charges does not completely correspond to a change in intensity of light.